Computerized tomographic (CT) scanners employ radiation from x-ray tubes. The radiation is focused on a target and the target is typically an arrangement of x-ray detectors that are positioned such that a tomographic image of one or more slices through a subject is reconstructed to produce an image.
The x-ray tube assembly typically operates with high voltage fed by control leads that pass through the housing into the tube. During operation, electrons are emitted from a source, usually a heated filament within a cathode, and accelerated to a focal spot located on the anode, or target. Upon striking the anode, x-rays are emitted from the focal spot as Bremstrahlung and characteristic radiation. The sources are typically high voltage sources. Such high voltage operation severely limits design aspects of the x-ray apparatus because it requires the high voltage to be insulated from other components of the x-ray tube. High voltage insulators are typically bulky and expensive.
In typical CT applications available today the x-ray tube and x-ray detector rotate on a gantry about three times per second around a patient located at the center of the gantry. Faster rotation speeds are desirable for imaging applications. For example, the motion of the heart can be effectively stopped if the information for an image can be obtained within a time period shorter than the time between two of the patient's heartbeats. However, rapidly growing centripetal forces due to increased gantry speed severely limit the tube's operation.
By contrast, in a stationary CT application, the x-ray source is a stationary arc source with distributed focal spots that can be activated by a control unit. The arc source would employ a large insulator to hold off the high operating voltage, which is on the order of 150 kV or larger. The insulator must be large which poses problems of cost, space, weight, and reliability concerns. A large insulator is very costly and very bulky adding considerable size and weight to the equipment.
To make the stationary CT source concept feasible, there is a need for reducing the cost and complexity of x-ray tubes and the arc source while generating high power x-rays.
In traditional x-ray tubes solid insulation is used to enable the generation of static electric fields for electron acceleration. Typically the cathode is at high negative voltage. For bipolar tubes this voltage is about −60 kV to −70 kV and for monopolar tubes this voltage typically ranges from −80 kV to −140 kV. However, applications employing voltages up to −200 kV are being discussed and lower voltages in the range of −30 kV are typical for mammography applications. For the higher electric fields more solid insulation is typically needed, thereby increasing the likelihood of failure under operation due to material defects. Failures of solid insulation are either surface flashovers or electrical breakdown in the bulk of the material. In both events the properties of the solid insulation are typically permanently changed, which requires the replacement of the x-ray tube.
Another disadvantage of solid insulation is the need to provide cathode supplies and controls on a high-voltage level. Examples are the filament drive supply, tube emission current controls and bias voltage supplies for electrostatic electron beam deflection. In each one of these examples at least one electrical feedthrough is required, that connects the signal from the high voltage end of the tube into the vacuum through the solid insulation. Generally feedthroughs increase the cost and complexity of the solid insulation and degrade the overall reliability of the solid insulation itself. Additionally, active electronic controls that are operated at high voltage levels to provide bias voltages are specifically susceptible to being damaged as a consequence of transient high voltage events, also called spits.
Another disadvantage of using dc electric fields in x-ray tubes, especially for CT, is the need for dual energy applications, which are of particular clinical value in differentiating cancerous tissue and benign calcification. In dual energy applications, two subsequent images are generated using electron beams at different cathode potentials. As an example consider alternating cathode potentials between −60 kV and −140 kV at a rate of 6 kHz. Due to limitations caused by the typical capacitive and inductive load of state-of-the-art generators, x-ray tubes, and connecting cable assemblies, such a square high-voltage waveform at 6 kHz cannot be achieved.